Multiple materials on one exported model in UE4?
If I understand you correctly, you want a mesh that has 2 different materials applied to it?
Then just model 1 mesh in blender. Select the faces that you want and assign them to "Material 1". Then select the other faces and assign them to "Material 2".

Export as an .fbx and import in UE4. UE4 will automatically make 2 material slots on your mesh. (They are called "elements")
Then you can just drag and drop one material into one slot and another material onto the other slot.